1. Field of the Invention
This invention relates to a data transmission unit used as a node, a number of which organize a ring ATM (Asynchronous Transfer Mode) network. The data transmission units connected together in a ring via transmission paths to transmit data by using packets called cells.
This invention is especially effective for an optional transmission unit to transmit multimedia information to another unit.
2. Description of the Related Art
One of conventional data transmission units organizing a ring ATM network is disclosed in "A study on the architecture of a high-speed ring network(ATMR)" (Ito, et al : Switching Systems Engineering, IEICE Technical Report).
This ATMR network system is, as shown in FIG. 1, a ring-type distributed node system organized by a number of data transmission units (access nodes : hereinafter referred to as ANs) connected together in a pair of rings which circulate data in opposite directions. Each AN is connected with terminals. Data are transmitted and received between terminals via ANs by ATMR cells passing round the transmission rings. The ATMR cell consists of a binary digit string having a format shown in FIG. 2. The format is composed of a 12 bit access control field (ACF), a 16 bit ring virtual channel number (RVCN) comprising an access node address (ANA) and a logical channel number (LCN), 4 bit undefined bits, an 8 bit header check sequence and the user information of 48 octets carrying the contents of data.
The ACF indicates whether the ATMR cell is occupied with effective user information or not.
Having data transferred from its terminal, the AN receives an available unoccupied ATMR cell from the transmission ring, occupies the cell with both the data and the ANA, and transmits it to the transmission ring.
On the other hand, having received an occupied ATMR cell, the AN transmits the cell as it is to a next AN unless it is bound for the AN itself. When it is, the AN transfers the information set in the ATMR cell to the terminal, surrenders the cell to get back to the transmission ring. Hereafter, data transmission is continued in the same way.
The following is a description of impartial cell access (fairness) among ANs which must be guaranteed when multimedia information is transmitted by applying the ATMR system to a videoconference system or to an image distribution system. The fairness means that each AN must carry out transmission according to the qualities of each transmission datum, such as reliability or allowed delay time.
The reason that the fairness must be guaranteed is as follows.
Each AN can carry out data transmission only when it has received an unoccupied ATMR cell. If a single AN continues to transmit data exclusively, causing the other ANs to have delay time, it may result in poor display of image data and the like which require to be transmitted with a limited delay time.
Thus, the function to guarantee the fairness is very important, which determines the performance of the whole system.
According to the ATMR system, the fairness is guaranteed by assigning the number of usable ATMR cells as a window size to each AN.
Provided with a counter to count the number of used ATMR cells, each AN surrenders data transmission after exhausted the assigned size of window. After having exhausted the window, the AN transmits any unoccupied ATMR cell it receives to a next AN even it has data to be transmitted. Consequently, any AN can begin to carry out data transmission at the latest after all the other ANs have exhausted their windows. When all the ANs have exhausted their windows, their counters are reset and all the ANs can resume data transmission.
Reset of the counters is carried out as follows.
Each AN sets its own node address in the ACF of all the ATMR cells it transmits while its window is remained. On the other hand, having already exhausted the assigned window, the AN transmits a received ATMR cell to a next AN without changing the contents of the ACF. When an ATMR cell setting in the node address of an AN comes back to the AN after passing round the whole transmission ring without being changed the contents of the ACF, that means the other ANs have exhausted their windows. When the AN also has already exhausted the window at the point it receives the cell, it checks if the address set in the received ACF is identified with its own node address. If it is, the AN transmits a special cell called a reset cell so that the other ANs can reset their counters. Consequently, each AN can resume data transmission for assigned window.
Thus, the number of ATMR cells used by each AN until the counter is reset is guaranteed by assigning a window size as above.
Furthermore, the window size may be assigned depending on the allowed delay time of each medium so that data which have limited delay time can be transmitted preferentially to guarantee the fairness among media.
The construction of one of the ANs is described as follows based on FIG. 3. As mentioned above, the ANs are connected together for data transmission in a pair of rings which circulate data in opposite directions. Since both transmission rings have the same construction for data transmission, the following description is referred to one of them.
Cell receiving unit 12 of AN 11 analyses the RVCN (ANA+LCN) set in the cell header of an ATMR cell received from one end 13a of transmission ring 13. If the ATMR cell is judged to be bound for the AN 11 (if the ANA is identified with the address of the AN 11), it is converted into a cell having a standard format and transferred to terminal 21 via receiving buffer 14.
Cell shifting unit 15, when the ANA of a received ATMR cell is identified with the address of the AN 11, shifts the ATMR cell to an unoccupied ATMR cell and transmits it to another end 13b of the transmission ring 13 according to the direction of state transition control unit 18 which is described later. This makes an ATMR cell be released at the destination AN and be occupied again by an AN positioned after the AN to improve the coefficient of utilization of the ring by increasing total throughput. On the other hand, when the ANA is not identified with the address of the AN 11, it transmits the cell to the end 13b.
Transmitting buffers 16 temporarily store the cells transferred from the terminal 21. Each of the transmitting buffers 16 corresponds to the priority of each cell according to its quality class in transmission, and each of the cells transferred from the terminal 21 is stored to the corresponding transmitting buffer 16.
Cell transmitting unit 17 converts cells stored in the transmitting buffers 16 into ATMR cells having the format shown in FIG. 2 according to the direction of the state transition control unit 18, and transmits them through the end 13b of the transmission ring 13 to a next AN 11.
The state transition control unit 18 shifts the state of an ATMR cell received from the end 13a, based on both the ACF and various internal conditions of the AN 11 such as the number of the cells stored in the transmitting buffers 16 waiting to be transmitted, the size of remaining window, the value indicated by a timer for checking the delay time of transmission which are all managed by the state transition control unit 18, and directs either the cell shifting unit 15 or the cell transmitting unit 17 to transmit data.
Each of the ANs 11 organizing the ATMR system has the same construction, and the communication in the whole system is under the distributed control of each AN 11 based on the ACF and other units in the cell header.
The ATMR system provided with double transmission rings can maintain the reliability of the network by the control of a managing node (center node) switching from a current use ring to a stand-by ring and loopback when a trouble arose.
The switching from a current use ring to a stand-by ring means that one of the two transmission rings (a current use ring) is switched over to the other transmission ring (a stand-by ring) when any trouble arose in the original ring, in order to prevent the operation of the network from interrupting.
For example, as shown in FIG. 4 (a), under the conditions that the right-handed transmission ring R is used as the current use ring and the left-handed transmission ring L as the stand-by ring, if a trouble arises at a point between AN-B and AN-C on the right-handed transmission ring R, the operation of the network can be continued by being switched over to the left-handed transmission ring L.
The loopback means turning back of signals by the ANs positioned both sides of the troubled point to continue the operation if troubles arose on both of the rings at the same time. For example, as shown in FIG. 4 (b), even if troubles arise between AN-D and AN-E on both the rings, all the nodes can continue to communicate by the AN-D and AN-E loopback.
Moreover, the loopback can be also applied to the case shown in FIG. 4 (a) by turning back signals at AN-B and AN-C.
The ATMR system described heretofore trying to realize a network having higher fairness and more throughput has some drawbacks as follows.
After an AN (AN-M) having a large number of data began to transmit data by using every unoccupied ATMR cells received, the ANs positioned between the AN-M and the destination AN(AN-N) can not carry out data transmission until the AN-M exhausts the window size.
When the AN-M has ended its transmission, ANs positioned both closer in lower stream to the AN-M are given higher priority of transmission.
An AN which exhausted the window can not carry out data transmission until all the other ANs exhaust their windows.
Thus, fairness among ANs is guaranteed in the entire number of transmittable ATMR cells until their counters are reset, but not in a comparatively short term and in the order of data transmission. Also, it is difficult to shorten the delay time to be guaranteed.
Furthermore, when a trouble arose, no ATMR cell can pass round the transmission ring until the operation of the network is restored by switching of the transmission paths or loopback. These restoring are directed by the center node which spends some-time to respond to a notice from an AN detected the trouble, causing an interruption of communication.